Sum of field on filtered records?

What is the best way to get a sum of a field across records that I want to filter first?

I am creating an expense log. Each record is a different expense. Each Record has a specific User_ID field in it. Each record also has a specific date. Say I wanted to get the sum of all the expenses from a specific user over the course of a specific time. How would I do that?

I was thinking a calculation field. What would I calculate exactly? I'd need to filter by User_ID, then by between a Start_Date and an End_Date (which I do already have fields for), AND THEN sum them.

This may be a simple thing, I've just never done it before. Can I do it in a Calculation or is this something where it is really a single row filtered portal that shows a sum?